1. Field of the Invention
The inventive concept relates to a semiconductor device, and more particularly, to an amplifier capable of operating with low power.
Push-pull amplification circuits including CMOS transistors are widely in use. A push-pull amplification circuit can drive either a positive or a negative current into a load. For example, a complementary pair of transistors may be used: one to sink current from the load to ground and the other to supply current to the load from a positive power supply.
Amplification circuits widely used as typical acoustic amplifiers are roughly classified into analog amplification circuits and digital amplification circuits. Examples of analog amplification circuits, in which linearity is important, include class-A, class-B, and class-AB amplification circuits. An example of the digital amplification circuits is a class-D amplification circuit.
In a class A amplifier, 100% of the input signal is used all of the time to generate the amplified output. Class A amplifiers may be more linear than other types, but they may also be inefficient. In a class B amplifier, 50% of the signal is used to generate the amplified output. While class B amplifiers may be more efficient than class A amplifiers, class B amplifiers may suffer from signal distortion. For example, if two class B amplifiers are used together to reproduce an input signal (each amplifying one half of the signal), the amplified output signal may be distorted if the two class B amplifiers do not transition smoothly to generate the amplified output signal. This type of distortion is called crossover distortion.
Unlike class A and B amplifiers, class AB amplifiers contain two active elements that each amplify the input signal over half of the waveform. However, an element still outputs a signal of a lesser magnitude, even during its “inactive” half of the waveform to reduce crossover distortion. In other words, since each active element continues outputting to a lesser extent in its “off” half of the waveform, a dead zone, or zone where both elements are nearly off, may be reduced or eliminated.
Class D amplifiers include switching amplifiers, or pulse-width modulation amplifiers, which convert an input signal into output “pulses” having higher voltages than the input signal. Class D amplifiers may be more energy-efficient than classes A, B, and AB. A user may select any of the above amplifiers to amplify an input signal based on desired characteristics including cost, distortion, energy-efficiency, availability, and desired linearity of an amplified output signal to an input signal.
2. Description of the Related Art
Amplifiers generally have at least two stages: an input stage to prepare a signal for amplification and an output stage to amplify the signal. An input stage may include input ports, filters, signal conditioning, etc. An output stage may include transistors, such as MOSFETS, and a power supply connection. When an input signal from the input stage enters the output stage, the output stage may amplify the signal by supplying a voltage from the power supply connection to increase the amplitude of the signal, for example.
Modern amplifiers may utilize transconductance to amplify an input signal. In other words, an amplifier may convert an input signal voltage into a current and output the current to the output stage of the amplifier. The output stage may then convert the current back into an output voltage amplified by the power supply. The current output from the input stage influences the slew rate of the amplifier. More specifically, the slew rate may be determined by the maximum current supplied to a compensating capacitor in an output stage of the amplifier. The slew rate, in turn, influences the frequency response of the amplifier, or the maximum allowable frequency of an input signal without distorting an output signal.
Some types of distortion that may degrade an output signal include crossover distortion, as discussed above, noise, harmonic distortion, clipping, and intermodulation distortion.
However, while supplying a larger current to a compensation capacitor may increase a slew rate of the amplifier, and thus its maximum frequency response, it may also increase the heat output, power consumption, and/or size of the amplifier.